Binding of RecA to anti-parallel poly(dA) \ub7 2poly(dT) triple helix DNA

Binding of RecA protein to conventional anti-parallel poly(dA). 2poly(dT) tripler DNA has been studied using flow linear dichroism spectroscopy. The association requires the presence of cofactor analog adenosine 5\u27-O-3-thiotriphosphate (ATP gamma S) and occurs with a rate similar to that for the association of RecA to double-stranded poly(dA) poly(dT) DNA. The binding of RecA to DNA stiffens the nucleotide chain, as evidenced from high orientation already at low shear rates, and the complex with tripler DNA appears to be at least as stiff as that with the duplex DNA. Therefore, the observation of a lower magnitude of the LD spectrum at 260 nm, in the triplex-RecA compared to the duplex-RecA complex, but retained magnitude of protein LD at 280 nm, indicates a markedly impaired orientation of nucleo-bases, possibly reflecting a perturbation by RecA on the third strand making its bases deviate strongly from perpendicularity. The circular dichroism spectrum: appearing immediately after dissociation of RecA by SDS, suggests an intact tripler structure, meaning that complexation with RecA has not dissociated the third strand. In conclusion, binding of RecA to tripler DNA does not modify the main organisation of the strands, but could affect the base-base interactions between them. Tilted bases could reflect a conformational change that RecA imposes also on the biological intermediate tripler structure to relax the base-base hydrogen bonding between the DNA strands